The present invention relates generally to pumps, and more particularly to a pump for pumping concrete or other similar moist flowable material containing aggregate, and in which it is important to maintain a substantially constant proportion of mix of component substances in the material delivered by the pump. Pumps of this general type have been proposed in the past, as illustrated by U.S. Pat. No. Re. 26,820 to Bennett, U.S. Pat. No. 3,659,970 to McElroy, and by the prior patents cited therein.